This invention relates generally to an apparatus and method for utilizing waste heat in a combined cycle power plant. More particularly, hot combustion gases are selectively combined with the waste exhaust gases through a diverter system to permit the temperature, pressure and quality of the steam generated in the combined cycle plant to be selectively controlled.
The simplicity and flexibility of gas turbine power plants have made them increasingly attractive to electrical utilities as a means for generating electrical power. The low capital cost, ease of operation and variety of fuels that can be accommodated in gas turbine plants are viewed as significant advantages generally not available in conventional boiler-fired power plants. In particular, the brief start up period associated with gas turbine power plants have made them particularly attractive as a supplemental electrical generating means during short periods of peak electrical loading.
When a gas turbine power plant is used on a sustained basis for the generation of electricity, however, differences inherent in the thermodynamic processes governing a steam power cycle and a gas turbine power cycle generally favor the use of a steam power cycle for sustained operation. For example, although the pressure of the exhaust gas leaving a gas turbine plant is approximately atmospheric, the temperature of the exhaust gases is still relatively high. Since no further expansion of the exhaust gas is possible, the heat in the exhaust gas is generally wasted, resulting in relatively low energy conversion efficiencies for the gas turbine plant. In contrast, a conventional steam power plant, when operated with a condenser, permits heat to be rejected at a temperature much closer to the surrounding environmental temperature, which results in a greater energy conversion efficiency.
The elevated exhaust gas temperatures generally associated with gas turbine power plants suggests that a gas turbine power plant may be combined with the advantageous features of a conventional steam power plant to achieve cogeneration in a combined cycle power plant, where the source of thermal energy for the steam cycle is provided at least in part by the hot exhaust gases from the gas turbine plant. As a result, considerable effort has been expended in developing methods to recover the available energy in the gas turbine exhaust flows through the use of combined cycle power plants, particularly where the gas turbine power plant operates on a sustained basis.
Referring now to FIG. 1, a combined cycle power plant 20 according to the prior art is shown. The combined cycle power plant 20 generally consists of a gas turbine section 18 that is coupled to a steam cycle section 19. The power plant 20 as shown in FIG. 1 is structured to utilize solid fuels, as will be discussed in further detail below. The gas turbine section 18 generally includes a compressor 2 that receives and compresses atmospheric air 1 and delivers the compressed air to a combustion chamber 3. The combustion chamber 3 also receives pulverized fuel material from a fuel pulverizer and conveyor 21 that, in turn, receives material from a fuel source 6. The pulverized fuel is directed to the combustion chamber 3 where it is mixed with the compressed air and burned with the pulverized fuel. The hot gases resulting from the combustion are routed to a cyclone separator 22 to separate fly ash from the combustion gases. The combustion gases are routed to a turbine 4, where they are partially expanded to recover sufficient mechanical energy to drive the compressor 2 through a power transmission shaft 5. The combustion gases are further expanded through a power turbine 24 that is coupled to an electrical generator 17 through a power transmission shaft 13. Electrical energy produced by the generator 17 may be supplied to an external electrical supply grid 23. Subsequent to the expansion of the exhaust gas in the power turbine 24, the gases 7 are routed from the turbine 24 to the steam cycle section 19, which generally includes a heat recovery steam generator (HRSG) 9 that receives the exhaust gas 7. Steam 14 is generated in the HRSG 9 when the latent heat of evaporation is transferred from the exhaust gas 7 to the feed water 15 within the HRSG 9. The exhaust gas 7 is then released to the atmosphere through a stack 8. The steam 14 thus generated is routed to a steam turbine 10 and expanded to recover mechanical energy. The steam turbine 10 is coupled to an electrical generator 16 by a transmission shaft 11. Electrical energy produced by the generator 16 may also be supplied to the grid 23. The steam 14 exhausted from the turbine 10 is routed to a condenser 12, and then returned to the HRSG 9 for reheating.
Still referring to FIG. 1, the steam 14 generated in the HRSG 9 preferably attains sufficient pressure and temperature to obtain acceptable efficiencies from the steam cycle section 19, and to minimize the moisture content in the steam 14. However, a particular shortcoming associated with the foregoing combined cycle plant 20 is that the temperature of the exhaust gas 7 frequently limits the temperature of the steam generated in the HRSG 9. Moreover, the exhaust gas temperature similarly limits the maximum pressure of the steam since the saturation temperature of the steam increases with its pressure and only the portion of the heat in the exhaust gas 7, which is above the saturation temperature of the feed water 15 in the HRSG 9, is available for the generation of steam. Unless the temperature of the exhaust gas 7 entering the HRSG 9 can be augmented, a lower thermal efficiency from the steam power section 19 is often encountered.
Reduced exhaust gas temperatures are particularly problematic in combined cycle plants 20 that employ biomass fuels, since the heating values of these fuels is significantly lower than heating values associated with hydrocarbon gases, coal or petroleum distillate fuels. Biomass fuels are defined as a solid fuel material of plant origin, consisting, for example, of wood chips or scrap residues, tree barks, or bagasse from sugar cane processing. As a result, the low temperature of the exhaust gas 7 prevents biomass combustion plants from using the exhaust gas energy for cogeneration. For example, U.S. Pat. No. 5,720,165 to Rizzie, et al., discloses a biomass combustion system that generates steam for injection into the power turbine of the gas turbine plant. The Rizzie patent does not disclose a system may be used in a combined cycle plant, as described above.
Other prior art systems have addressed the problem of insufficient exhaust gas temperatures in combined cycle plants by relying on sophisticated feed water management systems, and therefore do not propose augmenting the energy in the exhaust gases exhausted from the gas turbine plant. For example, U.S. Pat. No. 5,799,481 to Fetescu discloses that the performance of the steam power cycle portion of a combined cycle plant may be enhanced through a sophisticated feed water control system that uses an HRSG of complicated design. Similarly, U.S. Pat. No. 4,976,100 to Lee discloses that the exhaust gas energy of the gas turbine portion of the combined cycle plant may be more effectively recovered by allowing the exhaust gases, rather than steam exhausted from the steam turbine, to heat the feed water prior to entry into an HRSG. Accordingly, the prior art systems as disclosed in the Fetescu and Lee references are directed only towards more careful management of feed water heating, and cannot overcome the basic limitation of insufficient exhaust gas temperature inherent in combined cycle plants.
Thus, those concerned with the design and operation of combined cycle plants are highly aware of the need for a system that will permit the augmentation of the energy available in the gas turbine exhaust in a simple and convenient manner, thereby permitting the generation of steam of desired pressure, temperature and quality for various external steam consumers.
The present invention relates generally to steam generating systems in a combined cycle plant that is comprised of a gas turbine section that is operatively coupled to a steam cycle section to produce mechanical energy for electrical power generation, or for other purposes. In one aspect of the invention, a gas diversion valve is positioned between the gas turbine section and the steam cycle section of the combined plant to divert at least a portion of the hot combustion gases generated by the gas turbine section to the steam cycle section of the plant, thereby augmenting the heat available to the steam cycle section. In another aspect of the invention, steam produced within the steam cycle section of the combined plant is communicated to at least one of the turbines in the gas turbine section of the plant in order to augment the power output of the combined plant. In yet another aspect of the invention, the steam cycle section of the combined cycle plant may be selectively altered to allow the combined plant to accommodate the various tasks associated with sugar cane processing.